The growing global concern over persistent plastic waste has generated much interest in biodegradable polymers for everyday use. Biodegradable polymers based on polylactic acid (PLA) are one of the most attractive candidates as they can be readily produced from renewal agricultural sources such as corn. Recent developments in the manufacturing of the polymer economically from agricultural sources have accelerated the polymers emergence into the biodegradable plastic commodity market.
Linear acrylic copolymers have been disclosed for use as process aids in a blend with a biopolymer, such as polylactide. (US Application 2007-0179218). The disclosed linear acrylic copolymers do not provide satisfactory impact properties. Additives such as impact modifiers could be used in the polylactide composition.
One problem with many biodegradable polymers, such as polylactide, is the very brittle nature of the pure polymer. This property results in very low impact properties of finished articles, much lower than what is desirable for adequate product performance. Impact modifiers, such as methylmethacrylate-butadiene-styrene (MBS) and acrylic core-shell or block copolymers, are known to improve the impact properties of PVC and polycarbonate blends.
Block copolymers and core-shell polymers have been described for use in biodegradable polymers in PCT/US07/84502. This application is silent of particle size.
WO 2008/051443 describes clear impact modified polylactide resins. The resins are modified with bimodal core-shell impact modifiers, and the number average particle size of all particles and agglomerates in less than 210 nanometers.
Surprisingly it has been found that core-shell impact modifiers having a number average particle size of greater than 250 can be used in a biodegradable plastic, and still achieve excellent impact modification and low haze.